1. Field of the Invention
The present invention relates generally to metal oxide semiconductor (MOS) devices, and more particularly, to a mechanism for determining the respective effective oxide thickness of each of a plurality of dielectric materials forming a MOS (metal oxide semiconductor) stack.
2. Discussion of the Related Art
Referring to FIG. 1, in a MOS (metal oxide semiconductor) stack 100, a conductive structure 102 comprised of a metal (or other types of conductive material such as polysilicon for example) is formed on a high-K structure 104 comprised of a dielectric material having a dielectric constant higher than that of silicon dioxide (SiO2). An interfacial structure 106 is disposed between the high-K structure 104 and the semiconductor substrate 103 to provide a smooth structural transition from the high-K structure 104 to the semiconductor substrate 103. The high-K structure 104 has a first thickness T1, and the interfacial structure 106 has a second thickness T2.
For example, the conductive structure 102 is comprised of aluminum, the high-K structure 104 is comprised of a metal oxide, the semiconductor substrate 103 is comprised of silicon, and the interfacial structure 106 is comprised of silicon dioxide (SiO2). The high-K structure 104 comprised of a dielectric material having a dielectric constant higher than that of silicon dioxide (SiO2) is used as MOS device dimensions are further scaled down including the thickness of the dielectric materials between the conductive structure 102 and the semiconductor substrate 103. For a given capacitance, a dielectric material with a higher dielectric constant has a higher thickness.
When the high-K structure 104 is comprised of a dielectric material having a dielectric constant higher than that of silicon dioxide (SiO2), a higher thickness of the dielectric materials (including the high-K structure 104 and the interfacial structure 106) between the conductive structure 102 and the semiconductor substrate 103 is used than if simply silicon dioxide (SiO2) alone were to be used. A higher thickness of the dielectric materials between the conductive structure 102 and the semiconductor substrate 103 is advantageous for minimizing tunneling current through such dielectric materials. As MOS device dimensions are scaled down such that the thickness of the dielectric materials between the conductive structure 102 and the semiconductor substrate 103 is in a range of tens of angstroms, tunneling current may be a significant source of undesired leakage current for the MOS device.
With formation of the dielectric stack including the interfacial structure 106 and the high-K structure 104, determination of the respective effective oxide thickness of each of the interfacial structure 106 and the high-K structure 104 is desired in integrated circuit design. However, as MOS device dimensions are scaled down including the thickness of the dielectric materials 104 and 106 between the conductive structure 102 and the semiconductor substrate 103, conventional prior art techniques for measuring the thickness of the dielectric materials 104 and 106 of the MOS stack 100 may not be accurate.
Accordingly, in a general aspect of the present invention, the respective effective oxide thickness of each of a plurality of dielectric materials forming MOS stacks are determined electrically by forming a plurality of test MOS (metal oxide semiconductor) stacks.
In a general aspect of the present invention, in a system and method for determining a respective effective oxide thickness for each of a first dielectric structure comprised of a first dielectric material and a second dielectric structure comprised of a second dielectric material that form a MOS (metal oxide semiconductor) stack, a first plurality of test MOS (metal oxide semiconductor) stacks are formed. Each test MOS stack is comprised of a respective first dielectric structure comprised of the first dielectric material and a respective second dielectric structure comprised of the second dielectric material.
A respective deposition time for forming the respective first dielectric structure corresponding to each of the first plurality of test MOS stacks is varied such that a respective first effective oxide thickness of the respective first dielectric structure varies for the first plurality of test MOS stacks. A respective second effective oxide thickness of the respective second dielectric structure is maintained to be substantially same for each of the first plurality of test MOS stacks. A respective total effective oxide thickness, EOTMOS, is measured for each of the first plurality of test MOS stacks. A first graph having total effective oxide thickness as a first axis and having deposition time for forming the first dielectric structure as a second axis is generated by plotting the respective total effective oxide thickness, EOTMOS, versus the respective deposition time for forming the respective first dielectric structure for each of the first plurality of test MOS stacks. The respective second effective oxide thickness of the respective second dielectric structure that is substantially same for each of the first plurality of test MOS stacks is determined from an intercept of the first axis of total effective oxide thickness when deposition time for forming the first dielectric structure of the second axis is substantially zero in the first graph.
In another aspect of the present invention, the respective first effective oxide thickness of the respective first dielectric structure is determined for each of the first plurality of test MOS stacks by subtracting the respective second effective oxide thickness from the respective total effective oxide thickness, EOTMOS, for each of the first plurality of test MOS stacks.
In a further aspect of the present invention, an off-set to the second effective oxide thickness is determined by forming a second plurality of test MOS stacks. Each test MOS stack is comprised of a respective first dielectric structure comprised of the first dielectric material and a respective second dielectric structure comprised of the second dielectric material. A respective deposition time for forming the respective second dielectric structure corresponding to each of the second plurality of test MOS stacks is varied such that a respective second effective oxide thickness of the respective second dielectric structure varies for the second plurality of test MOS stacks. A respective first effective oxide thickness of the respective first dielectric structure is maintained to be substantially same for each of the second plurality of test MOS stacks.
A respective total effective oxide thickness, EOTMOS, is measured for each of the second plurality of test MOS stacks. A second graph having total effective oxide thickness as a first axis and having deposition time for forming the second dielectric structure as a second axis is generated by plotting the respective total effective oxide thickness, EOTMOS, versus the respective deposition time for forming the respective second dielectric structure for each of the second plurality of test MOS stacks. The respective first effective oxide thickness of the respective first dielectric structure that is substantially same for each of the second plurality of test MOS stacks is determined from an intercept of the first axis of total effective oxide thickness when deposition time for forming the second dielectric structure of the second axis is substantially zero in the second graph.
The respective second effective oxide thickness of the respective second dielectric structure for each of the second plurality of test MOS stacks is determined by subtracting the respective first effective oxide thickness from the respective total effective oxide thickness, EOTMOS, for each of the second plurality of test MOS stacks. The off-set is determined by comparing the respective first effective oxide thicknesses and the respective second effective oxide thicknesses determined for the first plurality of test MOS stacks and determined for the second plurality of test MOS stacks.
The present invention may be used to particular advantage when the respective total effective oxide thickness, EOTMOS, for each of the first plurality of test MOS stacks or of the second plurality of test MOS stacks is measured using a capacitance and voltage measurement system or a current and voltage measurement system.
In one embodiment of the present invention, the first dielectric material of the first dielectric structure is comprised of a high-K dielectric material having a dielectric constant higher than that of silicon dioxide (SiO2). In that case, the second dielectric material of the second dielectric structure is comprised of an interfacial dielectric material disposed between a substrate and the first dielectric structure. For example, the second dielectric material of the second dielectric structure is comprised of silicon dioxide (SiO2) when the first plurality of test MOS stacks are formed on a silicon substrate.
In this manner, the respective effective oxide thickness is electrically determined for each of the first dielectric structure and the second dielectric structure forming test MOS stacks. Such effective oxide thickness may be accurately determined even when the first and second dielectric structures are relatively thin in the range of angstroms.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.